Theoretical considerations on molecular transport in dialysis and sorbent therapy for uremia

The technology of uremic blood purification has grown rapidly over the past decade and has provided the clinician with a wide range of therapeutic options. These options involve mass transfer processes which may be primarily due to diffusion or convection, or a combination of both mechanisms. However, regardless of the mechanism of molecular transport, evaluation of the clinical utility of these therapies requires studies which provide sufficient data to solve the appropriate rate equations and to close mass balances. Data from the recent hemodialysis, peritoneal dialysis and sorbent literature are analyzed to show the magnitude of variability in both patient and therapy-related mass balance paramters for urea nitrogen (U) and middle molecules (MM) and to provide unambiguous comparisons for some of these therapies. A theoretical model is developed to describe sorbent-mediated gut elimination of solute as a first-order clearance limited by sorbent saturation. The model is used to analyze data in the literature on AL (OH)3 facilitated gut clearance of phosphate and indicates a gut P clearance of approximately 20 ml/min with maximum removal of approximately 800 mg P/24 hrs. Similar analysis of oxystarch indicated a gut U clearance of 2.5 ml/min and maximum removal of 1.5 gm/24 hrs.